This invention relates to an ion implantation apparatus and, in particular, to an ion implantation apparatus which is capable of producing an ion beam having no energy contamination.
In a recent ion implantation process for semiconductor device production, implantation energy has been getting lower and lower to make the depth of implanted ions shallower with scale reduction of micro patterning of the semiconductor device.
Extraction voltage of ion source is lowered to generate the low energy ions. However, the lower the extraction voltage is, the worse the extraction efficiency of ions from ion source is.
Further, the ions repel each other due to electric charge of themselves. This mutual repulsion causes a rapid increase of the ion beam diameter. This is what is called xe2x80x9cSpace charge effectxe2x80x9d. This effect becomes stronger with the decrease in ion energy. Consequently, transport efficiency is also reduced in low energy region. As a result, enough ion beam current can not be obtained.
To compensate for this difficulty, so called, post-deceleration technology has been used conventionally. In this technology, the ions are extracted from the ion source at a relatively high extraction voltage, and are mass analyzed and transported near to the target wafers, then the ions are decelerated down to the desired energy by a reverse electric field.
In such a deceleration method, there is an advantage that relatively higher beam current of low energy ions can be obtained easily. However, if the ions are neutralized by the reaction with residual gas before the deceleration position, such neutralized particles can not be decelerated by the reverse electric field. Consequently, such neutralized particles are implanted into the target wafer with original energy that is different from desired energy. This phenomenon is what is called xe2x80x9cEnergy contaminationxe2x80x9d.
Similar phenomena occur also in post-acceleration method in which the ions are accelerated by a forward electric field after mass analyzing to obtain higher energy ion beam.
If the ions are neutralized by the reaction with residual gas before the acceleration position, such neutralized particles can not be accelerated by the forward electric field. Consequently, such neutralized particles are implanted into the target wafer with original energy that is different from desired energy.
On the contrary, if the electrons of the ions are stripped more by the reaction with residual gas before the acceleration position, ions become higher valence (multi-charged) ions. Such multi-charged ions are accelerated by the forward electric field the valence times more than the single charged ions and implanted into the target wafer with different energy that is higher than the desired energy.
Thus, the energy contamination often occurs in the apparatus equipped with post-acceleration as well.
Referring to FIG. 1, description will be made about a related ion implantation apparatus equipped with post-deceleration or post-acceleration.
In FIG. 1, an ion beam extracted from an ion source 41 is mass-analyzed by a mass analyzing magnet 42 and a mass analyzing slit 43 to select desired ion species.
Specifically, immediately after the ion beam passes the mass analyzing magnet 42 at a point A, only desired ions exist on a trajectory that can pass through the mass analyzing slit 43.
In this case, energy of the desired ions at the Point A is determined in dependence upon the extraction voltage of the ion source 41 and the valence number of the ion. Therefore, the desired ions, which are on the trajectory towards the mass analyzing slit 43, have no energy dispersion at the point A.
After the ions pass through the mass analyzing slit 43, the ions are decelerated or accelerated in a post-stage electrode portion 44. In this event, the ions are decelerated or accelerated in accordance with a direction of an electric field applied to the post-stage electrode portion 44.
Namely, when a reverse electric field is applied, the ions are decelerated. On the other hand, when a forward electric filed is applied, the ions are accelerated.
The mass analyzing slit 43 is generally located nearby a downstream side of the post-stage electrode portion 44, hence an electrode part of the post-stage electrode portion 44 may perform the function of the mass-analyzing slit 43 in many cases.
As a specific example, description will be made about such a case that boron ions (B+) having one valance are implanted into a silicon wafer 46 in a wafer-processing chamber 45 with the energy less than 1 keV by using the post-deceleration.
In this event, a mode is classified into a first mode (drift mode) and a second mode (deceleration mode).
In the first mode, the ions are extracted from the ion source 41 with an extraction voltage less than 1 kV, and are implanted without the post-deceleration.
In the second mode, the ions are extracted with a relatively higher voltage (for example, n kV), and a reverse electric field is applied to the post-deceleration electrode portion 44 to finally produce the ion having energy less than 1 keV.
In the first mode, the extraction efficiency is degraded because the extraction voltage is low. Further, the transport efficiency is not high because the beam diverges by the space charge effect. Consequently, the beam current becomes small.
In the first mode, the energy contamination does not occur, however the beam current becomes small. In consequence, implantation time becomes long to implant the predetermined implantation quantity.
In the second mode, the current is increased in comparison with the first mode because of relatively higher extraction voltage than the first mode.
For example, when the ions are extracted from the ion source 41 with the voltage of several to 10 kV, the ions are transported towards the post-stage electrode portion 44 with the initial energy of several to 10 keV.
Further, the reverse electric field is applied such that the ions are decelerated down to energy of xc2xd-{fraction (1/10)} at the post-stage electrode portion 44 to finally produce the ion beam with the energy less than 1 keV.
However, a part of ions lose their charge by reaction with residual gas and become neutral particles in an area (an area B in FIG. 1) between the exit position (Point A) of the mass analyzing magnet 42 and a deceleration position.
In consequence, the part of the ions as the neutral particles are not affected by the reverse electric field for the deceleration. Thereby, the ions are implanted with the initial energy of several to 10 keV. As a result, not only the desired boron with the energy less than 1 keV but also the boron ions with the initial energy are inevitably implanted.
Thus, the beam current in the second mode of the deceleration is higher than the first mode, and the implantation time is advantageously short.
On the contrary, the particles not having the desired energy are inevitably mixed. This phenomenon is referred to as the energy contamination.
From the view point of getting the beam current as much as possible, deceleration ratio (namely, a ratio of energy before the deceleration to the energy after the deceleration) is to be desirably higher. However, as the deceleration ratio is higher, content or quantity of the energy contamination is generally higher.
FIG. 2 shows a typical example of implanted profile when the energy contamination takes place by the deceleration of the second mode in comparison with the profile of the first mode.
It is found out that the implanted profile of the second mode has higher energy component which is implanted to deeper position with the energy before deceleration.
To eliminate the components of energy contamination on the ion implantation apparatus with post-deceleration or post-acceleration, additional element for re-deflecting by electric field or magnetic filed has been conventionally used on the downstream trajectory of post-stage electrode.
In particular, this conventional method has been widely used on the apparatus equipped with the post-acceleration.
A structure of another related art is schematically illustrated in FIG. 3 to show the conventional method.
In FIG. 3, an ion beam extracted from an ion source 61 is mass-analyzed by a mass-analyzing magnet 62 and a subsequent mass-analyzing slit 63 to select desired ion species.
More specifically, immediately after the ions pass the mass analyzing magnet 62 at a point A, only the desired ions exist on such a trajectory that they can pass through the mass analyzing slit 63.
After the ions pass through the slit 63, the ions are decelerated or accelerated at a post-stage electrode portion 64.
With such a structure, neutralized particles and multi-charged ions, which are generated in the area B and have the different energy from the desired energy, are separated by a re-deflecting element 67 after post-deceleration/post-acceleration.
Consequently, only ions having the desired energy passes along a desired ion beam trajectory 68 to be introduced into a wafer substrate 66 in a wafer processing chamber 65.
In this case, a deflected ion beam having high valence passes along a trajectory 69a while a beam including neutral particles passes along a trajectory 69b. 
The related arts have the following disadvantages.
(1) The additional deflecting element is located at the downstream side of the post-deceleration/post-acceleration element. Thereby, an additional space is required, and a whole size of ion implantation apparatus becomes large.
(2) When low energy ions are obtained in the deceleration (namely, the second mode for the deceleration operation, the transport distance becomes long after the ions are decelerated to the low energy.
Thereby, the beam diverges by the space charge effect. Finally, the beam current decreases, and advantage is lost in the second mode of the deceleration operation.
It is therefore an object of this invention to provide an ion implantation apparatus which is capable of obtaining an ion beam having no energy contamination in both deceleration/acceleration modes of a post-stage.
An ion implantation apparatus according to this invention has an ion source, a mass-analyzing magnet, an accelerating/decelera-ting element, and deflecting elements.
With this structure, the mass analyzing magnet mass-analyzes an ion beam extracted from the ion source. The deflecting elements deflect the ion beam on the trajectory.
The deflecting elements are arranged between the mass-analyzing magnet and the accelerating/decelerating element.
In this case, the deflecting elements have a certain deflection angle each and have an operating mode and a non-operating mode. Each deflection angle is determined such that a final beam trajectory in a predetermined area before being introduced into a wafer substrate is matched to each other in both the operating mode and the non-operating mode.
The ion implantation apparatus further includes a separating slit arranged at the downstream side of the accelerating/decelerating element to eliminate undesirable particles causing energy contamination.
Further, the deflecting planes of the deflecting elements are substantially equivalent to the deflecting plane of the analyzing magnet.
In this case, deflection due to the deflecting elements are carried out by the use of electric field.
Alternatively, the deflection may be carried out by the use of magnetic field generated by an electromagnet preferably.
A beam trajectory is constituted such that the ion beam is transported to the wafer substrate when the post-acceleration/post-deceleration is not performed by the accelerating/decelerating element and the deflecting elements are not operated.
In the event, the deflecting elements are preferably composed of three stages such that the deflection operation is carried out three times at least. Thereby, the final beam trajectory in the predetermined area before being introduced into the wafer substrate is matched to the trajectory of non-operating mode.
Instead, the deflecting elements may be composed of two stages such that the ion beam is extracted from the mass-analyzing magnet on the trajectory offset from an original trajectory. In the event, preliminary deflection can be carried out by the mass-analyzing magnet.
The beam trajectory is offset by setting magnetic field of the mass-analyzing magnet weaker or stronger than a normal value.
The deflecting elements preferably include a first stage deflecting element and a second stage deflecting element. The mass-analyzing magnet deflects an ion beam trajectory around 90 degrees.
With such a structure, the first stage deflecting element is arranged as close to the mass-analyzing magnet as possible while the final stage deflecting element is arranged as close to the post-stage electrode portion as possible.
The accelerating/decelerating element is desirably arranged immediately after the mass-analyzing slit.
Further, the separating slit may be arranged at the predetermined distance from the accelerating/decelerating element.
In this condition, the mass-analyzing slit is substantially matched with a focal point of the ion beam through the mass-analyzing magnet.